Besides the electrolytic and bulb, the starter cap is very failure-prone.
In circuits where the filaments are used, it's in series with the output,
forming a series resonant tank; the low resistance of the cold filaments
draws lots of current, starting the tube quickly. When the tube starts
up, it appears in parallel with the cap, transforming the circuit into a
good old series inductance ballast.

The kind with only two pins skips the heating step and allows the tank
voltage and current to resonate even higher, until the tube breaks down
cold-cathode style. Once ignited, ion bombardment keeps the filaments
warm, keeping the reignition and operating voltages normal.

High voltage film caps are big and expensive, so understandably, they
don't like to use them very much. Often, a poor green (polyester) type is
found, which isn't even green anymore, but black from the abuse. Others
may be burned through, having experienced too many starts (too much peak
voltage) that self-healing has burned away most of the capacitance. The
circuit then either tries oscillating too high (burning itself from
switching loss) or oscillates lazily or latches (resonance no longer
strong enough to draw enough current to provide sufficient feedback).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

"P E Schoen" wrote in message
...
I have a desk lamp, with a magnifier and a 12W T4 circline fluorescent
bulb,
that I use constantly for working on PCBs and electronics projects in
general. I bought it several years ago and recently it started flickering
and then died. I replaced the bulb, but still no joy, and after replacing
the blown fuse and two damaged transistors, I found that the little
transformer had an open winding.

So, I thought, a 60W equivalent CFL is actually about 11-13 watts, and the
little circuit in them should work. I had a couple of broken or dead bulbs
ready for recycling, so I opened the bases, cut the leads, and extracted
the
PCBs. After a few unsuccessful tries, I was able to get it to work and now
my lamp is once again operational.

I found some schematics of the CFL driver boards he
http://www.pavouk.org/hw/lamp/en_index.html

Some of those circuits matched what I had almost exactly. It was a little
difficult to follow the explanation of how they operate, but what was
confusing is the four pins shown on the lamp itself, which is also how the
bulbs are made. I assume they are the heaters that are usually activated
with a starter, but I did not find any continuity on those pins. The desk
lamp only had one wire to each of two pins on the circline bulb, but in
the
CFLs all four wires were connected to different points on the PCB. It
would
only work when I shorted the connections that would have gone to the
heaters, and it seems to work very well. It starts to light at 50 VAC and
reaches full brightness at 100-120V, at which it draws about 100mA. That's
close enough to 12 watts for me!

Here's a little clip of my repair project:
http://www.youtube.com/watch?v=e7QJQ...ature=youtu.be

This is good to know. Those little circuits in each CFL have a lot of good
components, including a DIAC and high voltage transistors, and it's
probably
possible to use them to drive small fluorescent lamps for DIY projects or
repair. I doubt they could be used for the 40W tubes, but a driver from a
100W CFL should work on a 20W tube such as are in many desk lamps and
small
kitchen fixtures.

Does anyone know if there is any problem with this?

Thanks,

Paul

PS: GO RAVENS!!!!!